Allow overriding the config file location in Docker by setting the APP_CONFIG_FILE environment variable

[nvx]

This change sets the APP_CONFIG_FILE environment variable in the Dockerfile to set the default location of the config file rather than hard-coding it in the entrypoint script.

Benefits

This allows for specifying a custom path for the config file by setting an environment variable. This can be useful to work around issues like this hashicorp/nomad#4797 when using Nomad or consul-template to generate config files where a bind mount needs to be done on the folder rather than the file directly.

This also makes the behaviour of the Docker container more consistent with the behaviour when running outside a Docker container.

Possible Drawbacks

None, existing behaviour is retained unless the APP_CONFIG_FILE environment variable is overridden from the default value.

Applicable Issues

N/A

[dortegau]

Please, could you add your change to the CHANGELOG.md file as is described here: https://github.com/idealista/prom2teams/blob/master/.github/CONTRIBUTING.md#changelog?

Thanks in advance!

[nvx]

Please, could you add your change to the CHANGELOG.md file as is described here: https://github.com/idealista/prom2teams/blob/master/.github/CONTRIBUTING.md#changelog?

Thanks in advance!

Done!